Field of the Invention
This invention relates to tube plugs and more particularly to apparatus for plugging heat exchanger tubes.
In tube-type heat exchangers, a first fluid flows through the tubes of the heat exchanger while a second fluid surrounds the outside of the tubes such that heat exchange occurs between the two fluids. Degradation of some of the tubes may occur such that a leak either is impending or occurs therein which allows the two fluids to mingle. When this occurs it is sometimes necessary to plug the tubes so that the fluid does not flow through the tubes, thereby preventing leakage from the tubes.
In nuclear reactor power plants, the tube-type heat exchangers are commonly referred to as steam generators. If degradation occurs in the tubes of a nuclear steam generator to such an extent that the coolant in the tubes is allowed to mingle with the coolant outside of the tubes, a more significant problem arises. Since the fluid flowing in the tubes of a nuclear steam generator is generally radioactive, it is important that it not be allowed to leak from the tubes and contaminate the surrounding fluid. Therefore, when degradation beyond the permissible limit occurs in a nuclear steam generator heat exchanger tube, the tube is plugged so that the coolant is not permitted to flow through the tube. This prevents potential contamination of the fluid surrounding the tubes.
There are several kinds of plugs that may be used to plug the heat exchanger tubes. One such device used to plug heat exchanger tubes in nuclear steam generators is an explosive plugging device. With the explosive plugging devices, a metal plug containing an explosive charge is inserted into the heat exchanger tube. When the explosive is detonated, the plug is forced into close contact with the inside of the tube, thus blocking fluid flow through the tube. One problem associated with explosive plugging is that should it become necessary to remove the defective tube or the plug, the plug must be removed by time-consuming machining methods, which result in undesirable radioactive exposure and high cost.
There are also several kinds of plugs that can be used to plug heat exchanger tubes which do not use explosives. One such tube plug is described in U.S. Pat. No. 4,502,511, issued Mar. 5, 1985 in the name of Zafred, assigned to the Westinghouse Electric Corporation, and entitled "Tube Plug". The substantially cylindrical shell of this tube plug is expanded into contact with the inside surface of the tube, preferably by a conventional roller expander. The expander is then removed.
Other mechanical plugs are useful for plugging the heat exchanger tubes of nuclear steam generators. One such mechanical plug is described in U.S. Pat. No. 4,390,042, issued June 28, 1983 in the name of Kucherer et al., assigned to the Westinghouse Electric Corporation, and entitled "Tube Plug". The shell of this plug has a tapered inner surface against which an externally tapered expander member moves by the application of a pulling force and the motion of the expander member relative to the shell causes the shell to expand into contact with the tube. The expander has a self-locking trailing edge to prevent inadvertent unlocking of the plug. A plurality of lands, which are formed on the outside of the shell, are impressed in the wall of the heat exchange tube, causing the plug to be locked, thereby establishing a type of labyrinth seal along the inner surface of the heat exchange tube.
Installation of this type of mechanical plug may be accomplished using a plug installation apparatus such as the apparatus described in U.S. Pat. No. 4,369,662, issued Jan. 25, 1983 in the name of Rieben et al., assigned to the Westinghouse Electric Corporation, and entitled "Plug Installation Apparatus". A rod or drawbar of this apparatus captures the expander member of the plug and a force, preferably provided hydraulically, urges the drawbar downward, axially moving the expander member with respect to the shell and causing the shell to expand into contact with the inner surface of the heat exchange tube. The installation apparatus is then removed, leaving the plug within the tube.
If removal of this type is required, the preferred removal apparatus is described in U.S. Pat. No. 4,366,617, issued Jan. 4, 1983 in the name of Nansteil et al., assigned to the Westinghouse Electric Corporation and entitled "Plug Removal Apparatus". A rod holds the shell of the plug while a ram pushes the expander member relative to the plug to relieve the pressure on the plug. An activating cylinder can then be used to extract the plug from the tube by pulling.
Pushing the expander into the larger portion of the plug may not loosen the plug from tight contact with the internal surface of the tube. Subsequent plug removal by pulling may result in marking of the tube wall. What is needed is a new mechanical tube plug that is capable of being quickly and easily installed in a heat exchange tube and which can be easily removed therefrom while causing a minimum amount of marking of the tube wall.